hum

Any advice on this problem ..
My Echoplex seems to be inducing hum into my effects
processor (zoom 9020) which I have mounted above it in my rack. If I
completely disconnect the EP from the mains the hum stops, if it's
connected ,even though it may be turned off it still causes hum thru
the effects.
Is it an earth loop thang ? I'm on 240v here if that makes a diff ..
any suggestions ?
thanks,
Robert
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